The invention relates to a process and an apparatus, according to the corresponding claims, for rounding sheet-metal blanks.
In the state of the art, flat rectangular or square sheet-metal blanks e.g. for the manufacture of can bodies are rounded and the two edges extending axially and parallel with one another are then welded together, thus forming the sheet-metal blank into a cylindrical shell.
For the rounding, the sheet-metal blank is shaped into a curve in a rounding means, for example by being guided by a wedge to an inner bending roll and being passed immediately thereafter through a pinch point between the inner bending roll and an outer bending roll with these two rolls pressed together. This bends the initially flat blank about a constant bending radius so that the portion of the blank emerging from the pinch point no longer advances in a straight line in the original feed direction, but runs in a curve into a natural rounding position on the bending circle defined by the bending radius. In the case of can body manufacture, the curved blank passes essentially around the entire bending circle so that the blank is presented in the rounding position as an almost closed cylindrical shell. In practice the bending circle or rounding position may be slightly deformed by the weight and elasticity of the curved blank, but this can be prevented by providing guide means for the rounded portion of the blank.
The radius of the bending circle is determined by the radius of the inner bending roll, by the position of the wedge, and by the elasticity of the sheet material (sheet thickness and limit of elasticity). For can body manufacture, it is usually made somewhat smaller than the radius of the can body after welding, so that the rounded blank forms a cylindrical shell with the axial edges slightly overlapping.
The guide means for the rounded portion of the blank usually comprise an essentially cylindrical inner rounding mandrel and an essentially hollow-cylindrical outer guide, with the intervening space defining the circular slot lying on the bending circle. After rounding is complete, the shell is ejected axially from the circular slot, and another blank is inserted into the bending machine and into the circular slot.
The cycle time required for rounding a sheet-metal blank by the known process described above is made up of the time required for rounding and the time required for rejection. The rounding time is determined by the rate of feed and the length of the blank to be rounded. The ejection time is determined inter alia by the axial length of the rounded blank. In other words, the blanks have to be fed to the rounding means spaced apart at a minimum interval which is governed not by the rounding operation itself, but by the axial length of the rounded blank and by the ejection mechanism employed.